I. Field of the Invention
The present invention relates to a novel impulse, or drop-on-demand, ink jet print head and, more particularly, to optimal design and operating parameters for such a print head comprised of a plurality of plates held together in a superposed contiguous relationship.
II. Description of the Prior Art
Ink jet systems, and particularly impulse ink jet systems, are well known in the art. The principle behind an impulse ink jet as embodied in the present invention is the displacement of ink and the subsequent emission of ink droplets from an ink chamber through a nozzle by means of a driver mechanism which consists of a transducer (e.g., of piezoceramic material) bonded to a thin diaphragm. When a voltage is applied to the transducer, the transducer attempts to change its planar dimensions, but because it is securely and rigidly attached to the diaphragm, bending occurs. This bending displaces ink in the chamber, causing outward flow both through an inlet from the ink supply, or restrictor, and through an outlet or nozzle. The relative fluid impedances of the restrictor and nozzle are such that the outflow through the nozzle and restrictor are approximately equal. Fill of the ink chamber after a droplet emerges from the nozzle results from the capillary action of the ink meniscus within the nozzle which can be augmented by reverse bending of the transducer. Time for fill depends on the viscosity and surface tension of the ink as well as the impedance of the fluid channels. A subsequent ejection will then occur but only when fill has been accomplished and when, concurrently, the amplitude of the oscillations resulting from the first ejection have become negligible. Important measures of performance of an ink jet are the response of the meniscus to the applied voltage and the recovery time required between droplet ejections having uniform velocity and droplet diameter.
In general, it is desirable to employ a geometry that permits several nozzles to be positioned in a densely packed array. Since individual droplets normally have diameters of less than 100 micrometers, with 50 to 80 micrometers being typical diameters, it is normally desirable to produce an array of nozzles so that dots striking the surface can be connected together in a pattern. The diameter of an ejected droplet is typically of the order of the diameter of the nozzle that produces it. In order to write a connected line on a surface, it is common to use means such as a row of adjacent nozzles. It may also be desirable to arrange parallel rows of nozzles, with adjacent nozzles staggered so that one row fills the spaces left by a preceding row. In such an array, however, it is particularly important that the individual nozzles eject ink droplets of uniform diameter and velocity even at varying droplet ejection rates.
Past efforts to make drop-on-demand print heads have let to various approaches to solve the many problems that arise. It is necessary to decide upon the droplet diameter and velocity so as to achieve a desired resolution in printing. In general, it is easier to produce smaller droplet diameters at high ejection rates than larger diameter droplets. During the ejection process, the ink meniscus expands away from the nozzle until fluid instability causes the projecting fluid to cut off to a ligament that coalesces to a droplet and a remnant portion that draws back into the nozzle. Under conditions such as high velocity, the effect of this process is to form a main droplet and one or more satellite droplets which are typically smaller than the main droplet. The production of such satellite droplets may be an undesirable condition because the droplets may have speeds and directions that differ from the main droplet. Satellite droplets may also contribute to wetting at the nozzle. Wetting may lead to droplet misdirection or failure of emission.
In a properly operating drop-on-demand ink jet printer, a piezoelectric transducer is coupled mechanically to the chamber which bends the diaphragm. The fluid is substantially incompressible, and consequently air bubbles in the fluid within the chamber interfere with droplet ejection because air is compressible. As a result, it is necessary to eliminate or prevent air bubbles. The length of the nozzle and the operating voltage are chosen to prevent the ingestion of air.
The transducer must be designed so that the waveform is preferably one that is compatible with easily designed driver circuitry. The available voltage establishes the minimum dimensions of each transducer which, in turn, determines nozzle density. In general, in order to achieve the desired bending with the available voltage, a thin transducer is required.
The combination of a piezoelectric transducer bonded to a metal diaphragm was described by Stemme and Larsson in an article entitled "The Piezoelectric Capillary Injector--A New Hydrodynamic Method for Dot Pattern Generation," published in IEEE Transactions on Electron Devices, Volume ED-20, No. 1, January, 1973. One objective in the design of a driver is to facilitate a good mechanical coupling between the piezoelectric transducer and the diaphragm. This is typically done with a thin layer of an epoxy resin. It is also useful to minimize stress on the bond by matching flexibilities and thicknesses of the transducer and diaphragm so as to place a neutral zone of bending approximately at the plane of the bond.
Some representative examples of the prior art generally relating to ink jet print heads are noteworthy. U.S. Pat. No. 3,107,630 to Johnson et al is an early disclosure of the use of piezoceramic transducers being utilized to produce a high frequency cyclic pumping action. This was followed by U.S. Pat. No. 3,211,088 to Naiman which discloses the concept of an impulse ink jet print head. According to Naiman, when a voltage is applied to a transducer, ink is forced through the nozzle to form a spot upon a printing surface. The density of the spots so formed is determined by the number of nozzles employed in a matrix. Another variation of print head is disclosed in U.S. Pat. No. 3,767,120 issued to Stemme which utilizes a pair of chambers positioned in series between the transducer and the discharge nozzle.
Significant improvements over the then existing prior art are disclosed in a series of patents issued to Kyser et al, namely, U.S. Pat. Nos. 3,946,398, 4,189,734, 4,216,483, and 4,339,763. According to each of these disclosures, fluid droplets are projected from a plurality of nozzles at both a rate and in a volume controlled by electrical signals. In each instance, the nozzle requires that an associated transducer, and all of the components, lie in planes parallel to the plane of the droplets being ejected.
A more recent disclosure of an ink jet print head is provided in the U.S. Pat. No. 4,525,728 issued to Koto. In this instance, the print head includes a substrate having a plurality of pressurization chambers of rectangular configuration disposed thereon. Ink supply passages and nozzles are provided for each pressurization chamber. Each chamber also has a vibrating plate and a piezoceramic element which cooperate to change the volume of the pressurization chamber to cause ink to be ejected from the respective nozzles thereof.
In many instances of the prior art, ink jet print heads are assembled from a relatively large number of discrete components. The cost of such a construction is generally very high. For example, an array of ink jets requires an array of transducers. Typically, each transducer is separately mounted adjacent to the ink chamber of each jet by an adhesive bonding technique. This presents a problem when the number of transducers in the array is greater than, for example, a dozen, because complications generally arise due to increased handling complexities, for example, breakage or failure of electrical connections. In addition, the time and parts expense rise almost linearly with the number of separate transducers that must be bonded to the diaphragm. Furthermore, the chances of a failure or a wider spread in performance variables such as droplet volume and speed, generally increase. Additionally, in many instances, prior art print heads were large and cumbersome and could accommodate relatively few nozzles within the allotted space.
Typical commercial drop-on-demand ink jet print heads are available that produce droplets having diameters of the order of 80 micrometers at frequencies up to 3 KHz. Such print heads are typically designed to have an array of nozzles in one or more lines that spans a height of approximately 3.175 mm. In contrast, the printing of indicia or addresses on envelopes and the like presents some peculiar problems and opportunities. Their standard heights are approximately one inch and it is important to print them in one pass. Resolution is typically less important for this purpose than it is in general printing operations, and as a result, a droplet diameter of 70 to 80 micrometers is adequate. It is desirable to be able to print on objects moving at a velocity of 1.5 meters per second and to produce four dots per millimeter (100 dots per inch) from each nozzle. This requires a design frequency of at least 6,000 droplets per second from each nozzle, and an array of nozzles that is sufficiently numerous to span 25.4 mm (one inch).
One persistent problem that can interfere with successful and consistent operation of densely packed impulse or drop-on-demand ink jets is a phenomenon known as "crosstalk". This is the effect of the operation of one ink jet upon one or more other ink jets in an array, that is, from energy transfer from one ink jet to another ink jet. It may take place through a common ink source for two or more ink jet arrays that permits fluid coupling of a wave from one ink jet into another. Crosstalk may also result from the coupling of mechanical vibration through the solid structure of a print head. The effects of crosstalk through the solid structure are minimized by supplying sufficient rigidity to the structure. A number of systems have been employed to minimize crosstalk through the ink supply. In general, they involve the use of flexible tubing or other suitable structure to minimize the pressure wave that causes crosstalk.
The parent of the present application, namely, U.S. application Ser. No. 795,584 filed Nov. 6, 1985, is a distinct improvement over the prior art as just described and was conceived with knowledge of the prior art and the problems then existing. That application discloses an improved impulse ink jet print head and a method of fabricating such an improved print head. It comprises a plurality of superposed, contiguous plates including a nozzle plate with at least a pair of nozzles for ejecting ink droplets in a direction perpendicular to a plane of the plates. Another plate is a chamber defining at least a pair of coplanar axially aligned elongated chambers, each connected to a ink supply and having an outlet communicating with an associated nozzle. A diaphragm plate overlies the chamber plate and has transducers thereon for imparting a displacement of ink from each of the chambers to eject discrete ink droplets from the nozzles. Other plates may include a manifold plate for directing ink to a plurality of pairs of chambers and a restrictor plate with restrictors positioned between the ink supply and each of the chambers. The method of fabricating the print head includes forming the different plates, forming the transducers, and assembling all of the components in a particular relationship.
In short, it can be said that the invention disclosed in the parent to the instant application exhibits a significant advantage over earlier designs by providing a print head which is much more compact and which utilizes a reduced number of parts in its construction.